1. Field of the Invention
The invention relates to a laundry treatment apparatus, such as a washing machine, clothes dryer or washer-dryer with a drum with mounted for rotations about an at least approximately horizontal axis and with a drive motor arranged on the shaft of the drum and structured as a synchronous motor energized by a permanent magnet the stator of which is provided with a winding energized by a converter, the winding being structured as a single pole winding and the number of stator poles being different from those of the magnet poles.
2. The Prior Art
Washing machines of the kind referred to are generally known from WO-A-98/00902. Washing machines are also known from DE 3,819,651 A1 in which the laundry drum is driven directly without the use of the customary intermediate transmission (drive belt, pulley). In such drives the rotor constitutes the component for transmitting rotational movement to the drum of the washing machine. Furthermore, DE 3,819,861 A1 proposes to use an asynchronous motor with a squirrel-cage rotor. Such a motor is characterized by a relatively quiet movement, but it suffers from the drawback that because of the prevailing marginal conditions, such as, for instance, the large air gap and high pole construction in an asynchronous motor, good efficiency cannot be achieved. Yet in connection with a frequently operated household appliance an ecologically friendly, i.e. energy-saving operation, is desirable.
A motor for directly driving the drum has been described in DE 4.341,832 A1. That motor is structured as a synchronous motor fed by a converter. No further statements are made as regards the type of motor.
Furthermore, washing machines are known which are provided with directly driving motors structured as external rotor motors (DE 4,335,966 A1; EP 413,915 A1; EP 629,735 A2). The rotor may be manufactured as a deep-drawn component, such as a plastic bell or as a compound structure. The structure of a deep-drawn component is advantageous since in it, the iron forms the magnetic yoke and a hub may be integrated for receiving the bell. Among others, such a structure also constitutes an arrangement typical of venting motors.
Direct current motors without collectors are used in the above-mentioned direct drives for washing machines. See, for instance, WO-A-98/00902. The stator winding there described may be structured either as a conventional three-phase current winding with a winding pitch over several stator teeth or as a single pole winding with a winding around a stator pole. In this type of motor, commutation is performed by power semi-conductors. In such an arrangement, individual strands of the stator winding are energized by a d.c to a.c. converter in dependence of the stator position so that the excitation field rotates with the motor. In a treble stranded excitation winding current for the generation of torque flows at any given time in two strands only, the third strand remaining unenergized. The temporal current flow in the individual strands is block shaped or trapezoidal. For that reason, when switching the individual windings on and off, large current change velocities occur which generate noises at the motor. Such noises are undesirable, however, in laundry treatment apparatus of the kind sometimes installed in living facilities (kitchen, bathroom).
In electronically commutated d.c. motors, Hall sensors, magnetic transducers or optical sensors are utilized for sensing the rotor position. The mounting of such sensors and their appurtenant signal lines involves additional costs. Moreover, sensors and lines are subject to malfunctioning. A further drawback is that operating with field weakening is not easily accomplished in such self-controlled motors energized by permanent magnets. The large spread of torque and revolutions between washing and spinning operations necessary in washing machines usually results in large motor current spreads. For that reason, it is necessary to install switchable or tapped windings, or else the motor winding and the power semiconductors have to be sized for the largest possible current.
Synchronous motors sinusoidally energized and controlled by a converter are already known as servo-motors. They are utilized where precise positioning is required. In known servo-motors the stator winding is a conventional three-phase current winding, and the number of rotor and stator poles is identical. While the three-phase current winding is characterized by conventional and known winding techniques, the large amount of copper in the winding heads is a disadvantage as it not only increases manufacturing costs but also the structural depth of the motor. The latter aspect would, in washing machines with a housing of predetermined depth, reduce the volume of the drum. Moreover, for a controlled operation servo-motors require very accurate and expensive sensors for sensing the rotor position.
A further disadvantage of all previously mentioned motors with permanent magnet excitation is their lack of field weakening, since the magnetic flux of the motor essentially depends upon the field of the permanent magnets and is, therefore, constant. For washing machine drives such motors are, therefore, rather unsuited since a large spread of torque and revolutions between washing operations and spinning operations would entail a large spread of the motor current. The motor winding and the power semiconductors of the frequency converter would, therefore, have to be dimensioned for the largest current and would be very expensive. As an alternative, the windings could be tapped which would, however, require installing additional lines from the motor to the electronic components. Also, expensive switching relays would be required.
Therefore, it is the the object of the invention is to optimize, in a laundry treatment machine of the kind mentioned hereinbefore to optimize the motor in respect of energy consumption, low noise development and costs.
In accordance with the invention there is provided a laundry treatment apparatus having a drum rotationally mounted on a substantially horizontal axle and a synchronous motor with permanent magnets and a stator including winding strands energized by a frequency converter the output voltage of which is set such that continuous currents are generated in all strands.
In contrast to hitherto known direct drives for washing machines with d.c. motors without commutators, all three winding strands of the three-phase excitation winding are continuously energized in the drive concept here described, with the frequency of the excitation field being determined by the electronic control. In this case, the motor is operated as an externally controlled synchronous motor. In connection with a synchronous motor with permanent magnet excitation this method ensures that the noise developed is very low.
By utilizing a single pole winding, copper consumption is less than in a conventional three-phase current winding; the volume of copper of the winding heads is markedly less. Accordingly, the entire drive becomes smaller and more compact. Because of the smaller amount of copper and as a result of the lower copper losses higher degrees of efficiency can be achieved at the same motor size.
It is advantageous to structure the rotor as an external rotor. In this manner, the most compact structural shapes may be obtained because the torque generating radius of the air gap is located near the outer radius.
Furthermore, it is of advantage to utilize a control device which regulates the output voltage of the frequency converter by a control such that a minimum sinusoidal current is derived as a function of the load torque. Sinusoidal currents affect a very quiet motor movement and a reduction in losses resulting from current ripples. This is particularly true where the output voltage is set as a sinusoidal pulse width modulation. Moreover, the torque-dependent current control ensures an optimum degree of efficiency at each load point.
In synchronous motors with single pole windings the number of magnet poles characteristically deviates from the number of stator poles. A ratio of rotor poles to stator poles of 2 to 3 or of 4 to 3 is favorable in a treble stranded arrangement and continuous energization or in a rotational magnetomotive force of the stator winding. In these two cases only does the vectorial addition of the voltages induced in the individual pole windings yield a maximum and optimum degree of efficiency.
At a pole ratio of 4 to 3, the use of thirty stator poles is favorable in order to cover the required range of revolutions from 0 to 2,000 min. The selected number of poles ensures a definite start-up at an external control, low torque ripples and a large spread of revolutions.
Aside from this, it is advantageous to base the control device for controlling the motor current upon a mathematical model of the motor and to energize the winding strands without rotor position transducers. Since motor current and voltage at the motor may be detected at the frequency converter, there is no need for sensors at the motor.
In an advantageous embodiment of a control without sensors the mathematical model may be calibrated either as required or continuously. Motor-specific parameters such as winding resistance, motor inductance and the constant of the induced voltage may be detected by means of the current sensors and microprocessor control present in the frequency converter and the mathematical model may be adjusted on the basis of the measured values.
The essential advantage of the laundry treatment apparatus structured in accordance with the invention derives from the possibility of dimensioning the number of windings of the stator windings such that the level of the induced voltage or of the synchronous generated voltage for high revolutions is higher than the maximum output voltage of the frequency converter. Such a winding design makes possible a field weakening operation of the synchronous motor in the range of higher revolutions. The advantage of such a winding design is a marked reduction of the motor current in the washing mode. It may be selected in such a manner that the motor may be operated with the same current in the washing and spinning modes. Owing to the lower motor current smaller and less expensive power semiconductors may be utilized. Moreover, the losses in the power semiconductors are reduced so that the overall degree of efficiency of motor and power electronics is higher than in comparable drives utilizing the same quantity of copper. In order also to utilize field weakening when using a control with rotor position transducers, it is advantageous not to evaluate them at higher revolutions. At higher revolutions, large and short-term load deviations do not occur so that controlling the motor current is not absolutely necessary. In that case, the motor is operated with external controls with voltage and frequency being determined by the converter regardless of the position of the rotor field. The motor current will in such circumstances adjust itself within limits as a function of the load torque. In order to prevent an overload and an asynchronization of the motor, it will suffice to monitor the level of motor current as a function of the frequency of the rotational field.
Furthermore, It is also possible by field weakening to achieve good efficiency with high pole synchronous motors with permanent magnet excitation at high revolutions as the losses resulting from magnetic hysteresis are reduced as a result of field weakening.
Operation of d.c. motors without collectors with field weakening is possible only at great complexity as in such arrangements it would be necessary to change the position of the rotor position transducer or mathematically to shift the instants of commutation. For the above reasons, field weakening operation of servomotors is not known.